This invention deals with implantable pumps and in particular an improvement in implantable pump reservoir technology.
Within the art there are a variety of medical devices, implanted in living bodies, used for the purpose of dispensing at very low flow rates medication from a reservoir to a remote site. Typical of such commercial products are the INFUSAID Models 100 and 400 devices. These devices employ a two-phase fluorocarbon as a constant pressure driving media for purposes of delivering the medication from the pump reservoir to the delivery site.
The technology of constructing such fluorocarbon based two-phase pump reservoirs is well known. Representative in the art are U.S. Pat. Nos. 3,731,681, 4,525,165, 4,626,244 and 4,838,887. Those patents are representative of a host of systems known in the art which all utilize a fluorocarbon fluid such as FREON (Trademark-DuPont) which is hermetically sealed between a metallic housing and a flexible bellows or diaphragm. The material of choice for the housing and diaphragm is titanium. A variable volume space is thus defined opposite the fluorocarbon and constitutes a pressurized drug reservoir.
This reservoir is connected to a flow control device such as capillary tube, peristaltic pump, positive displacement pump, valve/accumulator/valve assembly or other fluid metering device employed in such implantable pump systems.
In use, the device is placed in a body cavity and thus heated by internal heat. The liquid/vapor equilibrium of the fluorocarbon forces drug from the reservoir through the metering system at a rate which is proportional to or relatively independent from the pressure difference across the system. For example, flow through a capillary tube restrictor depends on this pressure differential whereas flow through a valve/accumulator/valve assembly does not. The output of the metering system is fed through a delivery path for infusion at a specific implant site.
The pump is generally refilled by the use of a septum causing the bellows or diaphragm to deflect and thus essentially recharge the driving fluid by recondensation of the fluorocarbon vapor.
These devices in actual commercial use have proven to be both reliable in construction and operation. One disadvantage, however, is that the liquid/vapor pump is typically affected by the fluorocarbon pressure's extreme sensitivity to body temperature. Additionally, the vapor propellent system is configured to physically isolate the movable surface of the drug reservoir thus constraining the reservoir pressure to be independent of ambient conditions. The reservoir pressures in such implantable pumps are thus "absolute" rather than "gauge". Such pumps are unable to compensate for changes in fluid pressure at the delivery site. This is a problem when a capillary tube is employed to measure the fluid. Flow rates through such long capillaries may vary by as much as 40% as a function of temperature and atmospheric pressure changes. Such variations in flow rate are well documented in the literature. For example. U.S. Pat. No. 4,299,220 defines an accumulator/regulator arrangement in conjunction with a constant flow reservoir to compensate for variations in pressure and temperature.
Moreover, such systems are relatively expensive to construct. Not only is the cost of titanium very high, but the techniques required to hermetically seal the bellows/housing assembly make such pumps very expensive alternatives to other techniques of drug delivery.
One solution to these known deficiencies in prior art systems is set forth in U.S. Pat. No. 4,772,263. In accordance with the '263 patent, an implantable pump integrates the energy storage of a mechanical spring element with the pressure compensating capabilities of a movable reservoir surface. Consequently, a single element acts as a spring driven reservoir. Such a device maintains constant flow rate performance independent of temperature fluctuations and changes in atmospheric pressure.
However, one aspect of this system is that the volume of the spring material required to store pump energy be high with respect to the volume of infusate pumped. This requirement exists in such a system because the energy must be completely stored within the spring material and only a small fraction of the material can actually be used in a mechanically reversible manner (e.g., for metals) or the energy storage per unit volume is low (e.g., for elastomers). Such an inherent limitation in system design limits the design of an implantable pump system which requires minimal size yet at the same time maximum efficiency of the device volume.